This invention relates generally to combustors and more particularly, to methods and apparatus to facilitate decreasing combustor acoustics.
During the combustion of natural gas, pollutants such as, but not limited to, carbon monoxide (“CO2”), unburned hydrocarbons (“UHC”), and nitrogen oxides (“NOx”) may be formed and emitted into an ambient atmosphere. At least some known emission sources include devices such as, but not limited to, gas turbine engines and other combustion systems. Because of stringent emission control standards, it is desirable to control emissions of such pollutants by the suppressing formation of such emissions.
At least some known combustion systems implement combustion modification control technologies such as, but not limited to, Dry-Low-Emissions (“DLE”) combustors and other lean pre-mixed combustors to facilitate reducing emissions of pollutants from the combustion system by using pre-mixed fuel injection. For example, at least some known DLE combustors attempt to reduce the formation of pollutants by lowering a combustor flame temperature using lean fuel-air mixtures and/or pre-mixed combustion. However, at least some known DLE combustors experience combustion acoustics that can limit the operability and performance of a combustion system that includes such known DLE combustor.
Known strategies employed in an effort to reduce combustion acoustics include the following: (1) passive damping of pressure fluctuations with quarter-wave tubes, resonators, acoustic liners/baffles, and/or other acoustic damping devices; (2) incorporating design features into premixers to facilitate desensitizing a fuel-air mixing with respect to pressure fluctuations from a combustion chamber; (3) operating the combustor with significant variation in flame temperatures between individual domes of multidome combustors or individual premixers of singular annular combustors; (4) open-loop active control to introduce off-resonant fluctuations in fuel and/or air flows to facilitate weakening resonant modes; and/or (5) closed-loop active control methods that respond in real time to facilitate disturbing fuel and/or air flows in such a manner as to decouple physical processes responsible for feedback between pressure oscillations and heat release.
At least some known DLE combustors include both passive and active control features to facilitate suppressing combustion acoustics such as, but not limited to, combustion-inducing acoustic waves and combustion-inducing pressure oscillations that may be formed as a result of combustion instabilities that may be generated when a pre-mixed fuel and compressed air ignite. For example, quarter wave tubes have been used to passively damp pressure fluctuations adjacent to premixer inlets. Also, supplemental fuel circuits such as Enhanced Lean Blow-Out (“ELBO”) fuel circuits have been used in known pilot swirlers to actively inject smaller amounts of fuel into the combustor at a different location than a primary fuel injection location.
Compared to primary fuel circuits, ELBO fuel circuits generally require a shorter convective timescale for an ELBO fuel-air mixture to travel from a point of injection to a flame front where heat release occurs. As such, an acoustic frequency interacts differently with the ELBO fuel-air mixing at an ELBO fuel injection location as compared to primary fuel-air mixing at a primary injection location. As a result, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to pressure fluctuations in the combustor are generated to facilitate reducing combustion acoustics by reducing an amplitude of pressure fluctuations in the DLE combustor.
However, combustion of lean fuel-air mixtures generates heat temperatures that are sensitive to any variation in the fuel-air ratio of the fuel-air mixture. Such variations in the fuel-air ratio may be caused by fluctuations in a flow rate of the fuel and/or a flow rate of the compressed air. Because fuel flow and/or compressed air flow through known DLE combustors may be turbulent, fluctuations in the fuel and/or compressed air flow rates may cause pressure disturbances in a combustion chamber/zone of such DLE combustors. If such pressure disturbances interact with a fuel-air mixing process, any heat being released may also fluctuate to reinforce an initial pressure disturbance. Over time, the increased amplitude of pressure disturbances may cause damage to portions of the DLE combustor. As a result, operability, emissions, maintenance cost, and life of combustor components may be negatively affected.